Microbial community dynamics during high CO2 acclimation and potential of microalgae isolates in mitigation systems

Microalgae are promising platforms for CO 2 mitigation. However, high CO 2 exposure can impose physiological and environmental constraints, such as acidification and disruption of cellular homeostasis, which impair sensitive strains. In this study, we enriched a sample collected from a cement plant under 25% CO 2 (high carbon, HC) and different culture media (BG11 or MB3N), with parallel cultures under air (low carbon, LC). Over 15 years, metagenomics revealed simplified microalgae–bacterial consortia, in which CO 2 levels and culture medium shaped the prokaryotic component, while eukaryotes exhibited progressive homogenization. At the genus level, Ralstonia and Paraburkholderia dominated the HC cultures, along with Desmodesmus microalgae and Sarocladium fungi. Consortia growth under 25% CO 2 in both batch and continuous modes showed similar characteristics, but higher initial growth rates were observed in HC cultures, that suggest CO 2 adaptation. Microalgae isolates from HC consortia were Desmodesmus species with up to 75–100% CO 2 tolerance, which produced higher biomass than LC isolates. LC isolates were identified as unresolved species ( Scenedesmus or Coelastrum ), Desmodesmus , and Coelastrella with lower CO 2 tolerance, except for the Desmodesmus strain, which tolerated up to 100% CO 2 . High protein content (>50% d.w.), moderate carbohydrate levels (40%), low lipid levels (8–16%), and high photosynthetic pigments characterize the isolates at tolerable CO 2 levels. In addition, some strain-specific biomass traits were observed. Further studies should evaluate the role of prokaryotes in microalgae CO 2 mitigation systems. Overall, long-term acclimation produced extremely high CO 2 tolerant isolates with potential for CO 2 mitigation and value-added byproduct generation. • Metagenomics revealed long-term enriched cultures as microalgal-bacteria communities. • HC isolates were Desmodesmus species with up to 75–100% CO 2 tolerance. • LC isolates represented Scenedesmus/Coelastrum, Coelastrella , and Desmodesmus . • LC isolates showed lower CO 2 tolerance, except for Desmodesmus. • Isolated microalgae showed strain-specific biomass composition traits at high CO 2 .